1. Field of the Invention
The present invention relates to method and apparatus for performing measurements of the concentrations of electrolytes within solutions.
2. Description of Related Art
In recent years, ion sensors (especially, ion selective electrodes) have been actively applied to measurements in the medical field, and ions (e.g., sodium ions, potassium ions, and chlorine ions) dissolved in a sample such as blood or urine have been quantified.
Electrolyte measuring apparatus for measuring the concentrations of electrolytes (ionic concentration) within a sample such as urine or serum by the use of ion selective electrodes, i.e., apparatus employing a principle of measurement relying on ion selective electrodes, are known. Furthermore, a flow through analyzing system for measuring such plural ions at a time is available.
Such an electrolyte analyzer measures the electromotive force (i.e., the difference between the electric potential at one ion selective electrode and the electric potential at a reference electrode) produced by each sample through the use of the ion selective electrode (working electrode) and the reference electrode, measures the electromotive force produced by a reference fluid, and determines the concentration of the electrolyte of a measured component contained in the sample from measurement data obtained from the sample and reference fluid.
In the aforementioned electrolyte measuring apparatus utilizing the ion electrode method, a reference fluid and a sample are alternately measured in order to achieve stable electric potential measurements.
In this case, samples may be diluted with a buffer fluid prior to measurement in order to make uniform the samples in ionic strength and pH. At this time, the reference fluid that undergoes a measurement for comparison purposes is created using the same buffer fluid. A certain concentration of reference ions is contained in the reference fluid.
Two methods are conceivable as a method of adjusting the ion concentration of reference ions within the reference fluid.
The first method is to use a reference fluid concentration lower than that of the diluted sample solution. The second method is to use a reference fluid concentration close to the concentration of the diluted sample solution.
With the first method, a reference fluid adjusted to a low concentration provides a reference for measurements. Besides, measurements can be made after diluting each sample with the reference fluid. Consequently, the diluting fluid and reference fluid can be unified. However, a great difference exists between the sample concentration and the reference fluid concentration and so if a temperature fluctuation occurs, the output electric potential varies greatly. Therefore, there is the problem that even if a temperature correction is made, variations among individual electrodes due to their temperature characteristics cannot be eliminated.
The relationship between the temperature fluctuation and the fluctuation in the output electric potential is found from the Nernst equation. It is known that the output electric potential tends to increase in proportion to temperature.
In contrast, when the reference fluid is adjusted close to the concentration of the sample solution by the second method, the sample solution and the reference fluid produce almost equal amounts of output electric potential fluctuation in response to temperature fluctuation because of the Nernst equation. Therefore, the measurement is less affected by temperature. There is the advantage that the measuring accuracy is unaffected by temperature. For this reason, the second method is often used in actual apparatus.
In the second method, however, the concentration of the reference fluid is higher than the concentration of the diluting fluid. Therefore, in a system configuration which produces a sample solution by mixing a sample and a diluting fluid in a dilution vessel, if carry-over of the reference fluid occurs within the dilution vessel, there may be the problem that the concentration of a sample solution to be measured next will increase.
Such an undesirable situation becomes conspicuous when the concentration of the sample solution has decreased. This presents the problem that the linearity of measurement results on the low concentration side of sample solutions deteriorates. Furthermore, this issue becomes more conspicuous when the amount of the sample volume has been reduced.
The decrease in the issue is determined by the nature and quantity of the reference fluid that is the carry-over in the dilution vessel and, therefore, the following countermeasures (1)-(3) have been taken heretofore.
(1) The dilution vessel where a sample is diluted is made separate from the path through which the reference solution is supplied.
(2) The reference fluid left in the dilution vessel where the fluid is drawn in by a nozzle.
(3) The relative amount of carry-over reference fluid is made negligible by increasing the amounts of the sample and diluting fluid without taking any of the countermeasures (1) and (2).
In JP-A-6-109686, it is proposed that the amount of diluting fluid is managed during dilution within the dilution vessel. However, any countermeasure to be taken when there is a carry-over of the reference fluid is not proposed at all.
In JP-A-2009-19960, an apparatus is proposed in which the paths for the sample solution and reference fluid are made separate from each other in the same way as in the countermeasure (1) above and in which the reference fluid does not pass through the dilution vessel where the sample is diluted with a diluting fluid. In this apparatus, carry-over of the reference fluid in the dilution vessel is avoided but there arises another problem that the piping layout is complicated, thus complicating the apparatus.
Furthermore, if the countermeasure (2) using a suction means for aspirating the carry-over of the reference fluid in the dilution vessel is taken, there arises the new problem that the instrument is complicated.
If the countermeasure (3) consisting of increasing the amount of the sample solution compared with the reference fluid to make negligible the effects of the reference fluid without varying the configuration is taken, a larger amount of sample is needed than heretofore. This creates the problems that the burden on the patient or subject under test increases and that this countermeasure cannot be carried out when the amount of the sample is small.